Here is a function and command to see a diff between the currently edited file and its unmodified version in the filesystem. Just put this in your vimrc or in the plugin directory, open a file, make some modifications without saving them, and do :DiffSaved.

The following suggestion should work, but will only show you the output of the "diff" command on your system. It will NOT use Vim's excellent diff mode, where you can see side-by-side highlighted and folded differences between text in two different windows.